The present invention relates to an apparatus for measuring the voltage of an object of interest using an optical probe furnished with an electro-optic material which exhibits the electro-optic effect.
Apparatus have been known that measure the voltage at a given part of an object of interest such as an integrated circuit using an optical probe furnished with an electro-optic material such as LiTaO.sub.3 which exhibits the electro-optic effect (see, for example, U.S. Pat. No. 4,446,425 issued May 1, 1984; IEEE Journal of Quantum Electronics, Vol. QE-22, No. 1, pp. 69-78, published January 1986; J. A. Valdmanis, CLEO '78, pp. 352-353, published 1987; and LLE Review, July-September, pp. 158-163, published 1987).
FIGS. 10 and 11 show the principle for the method of voltage measurement described in these references. An optical probe 50 (or 60) shown in FIG. 10 (or 11) uses an electro-optic material 52 (or 62) cut out in such a way that the Z-axis of the crystal is parallel to the bottom end face 51 (or 61). In the system shown in FIG. 10, the bottom end face 51 of the electro-optic material 52 is further furnished with a total reflection mirror 53 formed of a dielectric multilayer film.
An object of interest 70 has both an electrode 71 for developing the voltage to be measured and an electrode (e.g. ground electrode) 72 for terminating the electric field lines coming from the electrode 71. In order to measure the voltage of the electrode 71 on the object 70, the optical probe 50 (or 60) is positioned as shown in FIG. 10 (or 11) and in such a way that the Z-axis of the electro-optic material 52 (or 62) will lie parallel to the line joining the electrodes 71 and 72. This arrangement enables detecting the intensity of an electric field (electric field lines) acting in a direction horizontal to the surface of the object 70. The refractive index of the electro-optic material 52 (or 62) varies in accordance with the intensity of a horizontal field, so in the case shown in FIG. 10, an incident light beam I.sub.1 that is launched along the central axial line B-B of the optical probe 50 is reflected from the total reflecting mirror 53; and in the case shown in FIG. 11, the incident light beam I.sub.1 is totally refracted three times by the inside surfaces of the optical probe 60. By extracting the change in the state of polarization of the light beam R.sub.1 emerging from each probe, the strength of the horizontal field is detected so as to measure the voltage developing at the electrode 71.
The prior art voltage measuring apparatus which employs the optical groove 50 or 60, however, has had the problem that theoretically it is only capable of measuring the field strength between the two electrodes 71 and 72 on the object 70, rather than the absolute voltage developing at the electrode 71.